Potential error in ventriculocisternal perfusion method for determination of cerebrospinal fluid formation rate in cats.

The cerebrospinal fluid (CSF) formation rate (Vf) has been extensively studied by the ventriculocisternal perfusion, a method still regarded as the most precise one. This method as well as the equation for the calculation of the CSF formation rate (Vf) was established by Heisey et al on indicator dilution in perfusate. They assumed that the dilution of the indicator substance in perfusion is a consequence of newly formed CSF i.e. a higher CSF formation rate would result in a higher degree of dilution of the indicator substance. Therefore, such method is indirect and any mistake in the interpretation of the degree of indicator dilution would lead to questionable and often contradictory results regarding CSF formation rates. According to Heisey's equation, Vf shoud not depend on the rate of ventriculocisternal perfusion. However it has been shown that Vf is perfusion dependt value, and also that during perfusion the indicator substance is partially absorbed into surrounding tissue. It is possible that obtained Vf dependence on perfusion rate was caused by observed absorption of indicator substances. For that reason, in anaesthetised cats ventriculocisternal perfusion was performed at higher (252.0 microL/min) and at lower perfusion rate (65.5 microL/min) and Vf was calculated at both experimental and corrected (just for absorbed amount) values of indicator substance. Since (inspite of the correction) the difference of 12.4 microL/min between lower (15.0 microL/min) and higher perfusion rate (27.4 microL/min) was obtained, it is obvious that ventriculocisternal perfusion method cannot be considered reliable for measuring CSF formation rate.